1. Field of the Invention
One embodiment of the present invention relates to an imaging element, an electronic appliance, a method for driving an imaging element, and a method for driving an electronic appliance.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Another embodiment of the present invention relates to a semiconductor device, a display device, a lighting device, a power storage device, a memory device, or a driving method or manufacturing method thereof.
2. Description of the Related Art
Charge-coupled device (CCD) imaging elements and complementary metal oxide semiconductor (CMOS) imaging elements are known as imaging elements that are mounted on mobile phones, smartphones, digital cameras, digital single-lens reflex cameras, and digital video cameras. In particular, CMOS imaging elements are more advantageous than CCD imaging elements because of their low power consumptions and high resolutions, and therefore, have been developed actively.
A rolling shutter system and a global shutter system are known as driving systems of CMOS imaging elements. In a rolling shutter system, pixels are sequentially exposed to light row by row. In a global shutter system, all the pixels are exposed to light at a time. The time of light exposure differs between pixels in a rolling shutter system; therefore, there is a problem in that a captured image is distorted in the case where the object is moving. Meanwhile, in a global shutter system, since all the pixels are exposed to light at the same time, a captured image does not have distortion even when the object is moving. Thus, a global shutter system is superior to a rolling shutter system as a method for driving a CMOS imaging element. However, in a global shutter system, the time of reading stored charge differs between pixels. Therefore, when leakage current is generated in a transfer transistor, charge which is generated in a photodiode cannot be retained, which causes a problem in that inherent information of the object is not reflected in an image. Against the problem, techniques for reducing leakage of charge with the use of a transistor including an oxide semiconductor as a transfer transistor are disclosed (Patent Documents 1 and 2).
In digital cameras and the like including CMOS imaging elements, digital image processing enables shooting modes such as long exposure and multiple exposure. In a silver salt film (analog) camera, long exposure corresponds to a shooting mode in which light exposure is performed while a shutter remains open for a long time. Long exposure is frequently used in taking an image of celestial objects because it makes it possible to take an image of the trajectory of a moving object. Multiple exposure corresponds to a shooting mode in which one image is obtained by superimposition of a plurality of images. The multiple exposure can provide an image which cannot be obtained by a normal shooting mode, which leads to creation of an image good in design. It is difficult to take an image while a shutter remains open for several hours in digital camera and the like. An image taken with a digital camera or the like is stored as image data and easily processed. Therefore, in shooting modes such as long exposure and multiple exposure, a process in which pieces of image data are prepared and synthesized to obtain one image data is generally executed.